A typical electrostatographic printing machine (such as a photocopier, laser printer, facsimile machine or the like) employs an imaging member that is exposed to an image to be printed. Exposure of the imaging member to the image to be printed records an electrostatic latent image on the imaging member corresponding to the informational areas contained within the image to be printed. Generally, the electrostatic latent image is developed by bringing a toner or developer mixture into contact therewith.
Two types of developer materials, liquid and dry, are typically employed in electrostatographic printing machines. A dry developer mixture usually comprises carrier granules having toner particles adhering triboelectrically thereto. Toner particles are attracted from the carrier granules to the latent image, forming a toner powder image thereon. Alternatively, a liquid developer material includes a liquid carrier having toner particles dispersed therein. The developer material is advanced into contact with the electrostatic latent image and the toner particles are deposited thereon in image configuration.
The developed image recorded on the photoconductive member is transferred to a support material such as paper either directly or via an intermediate transport member. The intermediate transport member may be in the form of a continuous belt or a roller. The toner image particles may be physically transferred by means of pressure or heat to the intermediate transport member, or they may be electrostatically transferred to the intermediate transport member by means of an electrical potential between the imaging member and the intermediate transport member. After the toner image has been transferred to the intermediate transport member, it is then transferred to the support material.
In electrostatographic printing machines in which the toner image is electrostatically transferred by a potential between the imaging member and the intermediate transport member, it is critical that the transfer of the toner particles to the intermediate transport member and the retention thereof be as complete as possible and that the image ultimately transferred to the support material have a high resolution.
U.S. Pat. No. 5,028,964 (Landa et al.) discloses an intermediate transport member in a system which uses heat and pressure to transfer an image to a substrate. The intermediate transport member may be a belt which is comprised of an arrangement of electrical conductors, wherein an angularly delimited portion of the intermediate transport member is energized. The energized portion is selected so as to roughly correspond with a region of a nip between the intermediate transport member and a photoconductor surface. The energized portion corresponds to the region filled with liquid, which is delineated by adjacent radii, thus reducing or eliminating electrical discharge. The intermediate transport member is comprised of a high tensile substrate, a resilient layer, a resistive heating layer containing a nickel-chrome alloy, and an insulative layer containing polyurethane. The electrical conductors are supported on the insulative layer. There is no discussion of a compliant or self-discharging conductive or semi-conductive outer layer comprising the intermediate transport member. Consequently, there could be a charge buildup on the insulative layer.
U.S. Pat. No. 4,984,025 (Landa et al.) discloses an intermediate transport member similar to that of U.S. Pat. No. 5,028,964 but with parallel arrays of uniformly spaced, energizable, electrical conductors supported on the insulative layer and embedded in a layer of conductive material such as a silicon-polyurethane copolymer. The parallel array permits the heating of the intermediate transport member to be spatially selective to enhance the cohesiveness of an image to be transferred from the intermediate transfer member to a substrate such as paper. As in U.S. Pat. No. 5,028,964, a charge buildup can result on the insulative layer.
U.S. Pat. No. 4,796,048 (Bean) discloses an apparatus which transfers a plurality of liquid images from a photoconductive member to a copy sheet. The apparatus may include an intermediate transport belt to transfer a toner image to a copy sheet with the use of a biased transfer roller. The intermediate transport belt has a smooth surface, is non-absorbent, and has a low surface energy.
U.S. Pat. No. 4,708,460 (Langdon) discloses an apparatus for transferring a liquid image from a photoconductive member to an intermediate transport member. The photoconductive surface is preferably made from a selenium alloy with a conductive substrate made from an aluminum alloy which is electrically grounded. A voltage source is coupled to a roller so as to electrically bias the roller with a suitable potential and polarity. This electrical bias, in turn, electrically biases the intermediate transport belt. The intermediate transport belt is positioned to contact a photoreceptor belt in a nip. Preferably, the intermediate transport belt is made from a somewhat electrically conductive silicone material having an electrical conductivity of about 10.sup.9 ohm-centimeters. Consequently, the belt is semiconductive. There is no indication that the belt is compliant, nor is there any indication of the belt comprising a dimensionally stable substrate.
U.S. Pat. No. 4,430,412 (Miwa et al.) discloses a method and apparatus for transferring and fixing a toner image, wherein a toner image is transferred and fixed onto a transfer material by an intermediate transfer member. The intermediate member may be a belt-type member which is pressed onto an outer periphery of a toner image retainer with a pressure roller. The intermediate member is suspended on a heat roller and a tension roller beside the pressure roller. The intermediate member is formed with a laminate of a transfer layer comprising a heat resistant elastic body such as silicone rubber or fluororubber, and a heat resistant base material such as stainless steel.
U.S. Pat. No. 3,893,761 (Buchan et al.) discloses a xerographic heat and pressure transfer and fusing apparatus having an intermediate transfer member which has a smooth surface, a surface free energy below 40 dynes per centimeter and a hardness from 30 to 70 durometer Shore .ANG.. The transfer member, preferably in the form of a belt, can be formed, for example, from a polyamide film substrate coated with 0.1-10 millimeters of silicone rubber or a fluoroelastomer.
In multicolor copying systems in which the toner image on the imaging member is transferred across an electrical potential to the intermediate transport member, the toner on the intermediate transport member has a tendency to change its charge sign, thereby neutralizing the electrical potential between the imaging member and the intermediate transport member.
Toner, which generally has a negative charge, is positioned on the imaging member after development in a multicolor photocopier. A charged biased transfer roller charges the back side of the intermediate transport member. Alternatively, a corona supplies a charge, normally positive, to the back side of an intermediate transport member. The positive charge on the back side of the intermediate transport member breaks down the air between the image receptor and the transport member and both the negative charges of the air and of the toner particles are attracted to the front side of the intermediate transport member. When the imaging member rotates and is recharged with another color of toner particles, the imaging member with the negatively charged toner particles is again positioned across from the intermediate transport image where the first toner was deposited. There are, however, already negatively charged toner particles on the front side of the intermediate transport member. The potential between the negatively charged toner particles on the imaging member and the positive charge on the back side of the intermediate transport member, in addition to the negatively charged particles on the intermediate transport member, again causes breakdown of the air. Positively charged particles of air are attracted to the front side of the intermediate transport member, due to the negatively charged toner particles already positioned on the intermediate transport member, thereby neutralizing the electrical field and preventing the migration of additional toner particles of different colors. To avoid this problem, it is presently necessary to use conductive electrostatic devices to discharge the intermediate transport member.
Each of the systems discussed above attempts to improve the quality of the image being electrostatically transferred. However, each of these systems has their limitations. Transport materials comprised of rubber are not very compatible with Isopar.RTM., a decane toner dispersant used for liquid toners. Insulative materials used for the transport member do not adequately discharge. In some cases, materials used for the surface of the intermediate transport member do not allow for the transfer and printing of a high resolution image to the copy sheet.